Delignification and bleaching of cellulose pulp layers with oxygen gas

ABSTRACT

A method of dividing a stream of pulp into a series of layers comprising discrete batches and progressively transferring the batches from layer to layer in controlled fashion so that the height of each layer does not exceed a maximum value at which the pulp at the bottom of a layer has a predetermined minimum gaseous content, the pulp being contacted with oxygen gas under pressure while progressively transferring pulp from one layer to the next in the series. The method may occur in an apparatus comprising a cylindrical pressure vessel including axially spaced floors which define chambers between them, the chambers being subdivided into a plurality of compartments by dividing walls disposed axially to the pressure vessel and transversely to the floors, each floor having an aperture and the compartments and floors being relatively movable to allow pulp in the compartments sequentially to be transferred from one chamber to the next as relative movement occurs.

Verreyne et a1.

DELIGNIFICATION AND BLEACHING OF CELLULOSE PULP LAYERS WITH OXYGEN GASlnventors: Abraham Jacob Verreyne, Petersfield; Leonard Austin Job,Bramley North, both of Republic of South Africa; Paul Rerolle,Nogent-sur Marne: .Iohan C. 1". C. Richter,

St. Jean Cap Ferrat. both of France Assignees: South African Pulp andPaper Industries Limited; LAlr Liquide, Societe Anonyse pour LEtude etLExploitation des Procedes Georges Claude and Aktiebolaget Kamyr Filed:July 10, 1969 Appl. No.: 840,639

Foreign Application Priority Data July 11, 1968 Sweden ..9540/68 July15, 1968 Sweden... ..9689/68 [451 May 2, 1972 [56] References CitedUNITED STATES PATENTS 1,529,919 3/1925 Richter ..l62/65 X 2,431,47811/1947 Hill l 162/1 7 3,298,900 l/l967 Laakso 162/17 3,492,199 l/197OKindron et al 162/71 2,662,821 12/1953 Muench 162/237 PrimaryExaminer-S. Leon Bashore Assistant Examiner-Arthur L. CorbinAn0rneyWaters, Roditi, Schwartz & Nissen [57] ABSTRACT A method ofdividing a stream of pulp into a series of layers comprising discretebatches and progressively transferring the batches from layer to layerin controlled fashion so that the height of each layer does not exceed amaximum value at which the pulp at the bottom of a layer has apredetermined minimum gaseous content, the pulp being contacted with oxygen gas under pressure while progressively transferring pulp from onelayer to the next in the series. The method may occur in an apparatuscomprising a cylindrical pressure vessel including axially spaced floorswhich define chambers between them, the chambers being subdivided into aplurality of compartments by dividing walls disposed axially to thepressure vessel and transversely to the floors, each floor having anaperture and the compartments and floors being relativelymovable toallow pulp in the compartments sequentially to be transferred from onechamber to the next as relative movement occurs.

11 Claims, 16 Drawing Figures PATENYEDHAY 2 1912 SHEET 2 a? 3 9 J 4 7 Mw m X 0 m 1 a 6 4 jh enmnmr EA? w iwer 1 5 7 m .a 4 2 (W w o 3 A, i; 5 gt? M DELIGNIFICATION AND BLEACHING OF CELLULOSE PULP LAYERS WITH OXYGENGAS This invention relates to the delignification and bleaching ofcellulose pulps with oxygen gas under pressure.

For the purpose of this specification the term oxygen gas" includes anygas containing free oxygen, such as air.

it is known that a pulp can be delignified and bleached by subjecting itin an alkaline medium to the action of oxygen gas. French Pat. No.1,387,853 discloses the treatment of a chemical pulp with oxygen gasunder pressure in an alkaline medium in the presence of a catalyst orprotector acting to preserve the physical and mechanical strengthproperties of the pulp. South African Pat. No. 67/3680 discloses amethod of delignifying and bleaching an alkaline pulp by pretreating thepulp with an acidic medium and thereafter subjecting the pulp in analkaline medium to the action of oxygen gas under pressure.

Delignification and bleaching agents other than oxygen gas are known andnormally such other agents react with lignins in an aqueous medium.Generally the correct amount of these other agents can be dissolved inthe aqueous phase of the wet pulp. Oxygen gas treatment must, however,be conducted under pressure and even then only small amounts of oxygencan be dissolved in water at such high oxygen partial pressures. As aresult, it is necessary for the oxygen gas to be applied directly to thepulp mass in sufficiently large quantity if a satisfactory reaction isto be obtained.

it has been found that the amount of oxygen required for a satisfactoryreaction can be accommodated by the pulp only if the bulk density of thepulp is sufficiently low and if substantially no free liquid drains fromthe pulp. The presence of drained liquid is detrimental in that itaccumulates in certain areas where it occupies space that ought to beoccupied by oxygen and excludes further oxygen from re-plenishing thatwhich has been dissolved and then consumed. Drainage of liquid alsoresults in unequal distribution of the alkali normally added andcorresponding variation in the final bleaching of the pulp.

It has further been found that the design of conventional ulp bleachingtowers are not satisfactory for the treatment of pulps with oxygen gas.Normally, such towers operate at relatively low consistencies with nofree gas occluded in the pulp. Tests have shown that if such towers areoperated at higher consistencies the pulp compresses under its ownweight and liquid is inclined to collect at the bottom of suchconventional towers so that the oxygen content of the pulp towards thebottom of such towers is too low for satisfactory reaction.

Although methods of repeated exposure of pulp to a gas are known suchmethods generally suffer the disadvantage of complex mechanisms withconsequent excessive wastage of space between stages and difficulty ofsealing against over pressure as well as high equipment costs.

Methods using a series of trays provided with rake arms causing transferalternatively at the periphery and the center of the trays are known.These suffer the disadvantage that energy is used wastefully due torollback of free flowing materials while sequential flow and control ofresidence time is poor as a consequence. The applicants have found byexperiment that this type of apparatus is incapable of handling beds ofpulp deeper than 0.4 meters in a manner capable of achieving goodresults by the oxygen bleaching process. As a result of this heightlimitation either the number of plough systems must be increased or thediameter of the pressure vessel must be increased. Both of these actionscause a high cost penalty.

It is accordingly an object of the present invention to provide improvedtreatment of cellulose pulp with oxygen gas under pressure with whichthe above disadvantages are at least minimized.

According to one aspect of the invention a method of treating acellulose pulp with oxygen gas under pressure includes the steps ofconstraining the pulp for movement along a prescribed path, contactingthe pulp with oxygen at a pressure in excess of atmospheric pressureduring at least part of its movement along the path, dividing the pulpinto a series of layers across the path, the height of each layer notexceeding a maximum value at which the pulp at the bottom of a layer hasa predetermined minimum gaseous content; and progressively transferringpulp after a controlled residence period from one layer to the next inthe series without increasing the height of any one layer beyond themaximum value.

Preferably the layers are located one above the other and the pulp istransferred from each layer to the next layer in the series under theinfluence of gravity.

The invention also includes the steps of introducing pulp at a highconsistency of between 16 and 67 percent to the prescribed path andfluffing the pulp prior to dividing it into said series of layers.

The minimum amount of oxygen required at the bottom of a layer willdepend on the number of layers in the series and the period of time thepulp is retained in each layer and may also depend on the temperatureand pressure.

The pulp may be transferred continuously, but preferably the pulp istransferred in stepwise fashion from one layer to the next in the seriesand away from the last layer in the series.

Further according to the invention the layers are sub-divided in to aplurality of discrete subdivisions or batches. Preferably the batchesare transferred from one layer to the next in a proper sequential mannerwithout intermixing with other batches.

Pulp may be transferred away from the final layer in the series,re-united and discharged at a predetermined rate.

Preferably each quantity of pulp to be transferred through subsequentlayers is determined towards the top of the path.

An advantage of such stepwise transfer of pulp is that the pulp mayrepeatedly be transferred through an atmosphere of oxygen gas, therebyto re-introduce further oxygen.

The pulp may preferably be contacted with oxygen at a partial pressurein excess of 3 bars absolute.

In a preferred arrangement, the pulp is contacted stepwise with oxygensubstantially all the way along the said path. Preferably the pulp iscontinuously fed to and removed from the prescribed path.

The applicants have found that for satisfactory reaction between thepulp and the oxygen gas, the following are important,

a. The pulp should be at a consistency which is sufficiently high forthe pulp to contain enough oxygen and so that there is no substantialdrainage of liquid from the pulp;

b. The pulp is in a well divided form, such as in the form of noodles,or preferably in fiufied form since the liquid retention of flutfed pulpis better than that of noodle pulp and the bulk density of the former islower than the latter;

c. The height of any pulp layer is not too great otherwise the pulpcompacts under its own weight with a resultant expulsion of liquidand/or oxygen at the bottom of the layer.

The applicants tests have indicated the following:

a. That water begins to drain from softwood noodle pulp at a consistencyof 15 percent under zero pressure and at a consistency of 18 percentunder a pressure of a 2.40 meter high pulp layer.

b. The water begins to drain from hardwood noodle pulp at a consistencyof 13 percent under zero pressure and at a consistency of 16 percentunder the pressure of 2.4 meter high pulp layer.

c. That fluffed pulp is less inclined to drain water.

d. That for satisfactory reaction between pulp and oxygen gas, the pulpshould contain at least 1 percent oxygen by weight on a dry pulp basisat the bottom of a layer of pulp. This condition is attained withdifferent pulps with the following maximum layer heights under aselected typical reaction condition of l 70 p.s.i.g. and C and usingpure oxygen:

Softwood noodle pulp at 20% consistency 3.00 meter. Softwood flufi'edpulp at 18% consistency 3.00 meter. Hardwood noodle pulp at 18%consistency 3.00 meter. Hardwood fiulfed pulp at l8% consistency 3.00meter.

The heights of these layers can be increased if the sidewalls exert anyfriction. However, the object is not to unnecessarily increase thefriction.

In the light of the above results, the method of the present inventionis preferably carried out with pulp at a consistency of not less than 18percent in the case of hardwood noodle and fluffed pulps and of softwoodfluffed pulp and with a pulp consistency of not less than 20 percent inthe case of softwood noodle pulp. Fluffed pulp is to be preferred.

Furthermore, the heights of the various layers may be arranged so as notto exceed 3.00 meters and preferably not to exceed 1.20 meters,especially if lower operating pressures are used.

According to another aspect of the invention, pulp treating apparatuscomprises a pressure vessel including a series of pulp trapping chambersso connected as to provide a pathway for the pulp to movegravitationally from chamber to chamber, at least one dividing wallprojecting upwardly transverse to the floor of each chamber to dividethe chamber into a plurality of compartments, means to permit relativemovement of the floor and the dividing wall so as to effect to allow thewall to sweep over the floor about an axis, an inlet port for pulp toenter the chamber and an outlet port therefrom in the floor, the outletport being characterized in a radial dimension substantially equal tothe effective radial dimension of the chamber, and means to permitoxygen gas to contact the pulp during passage along the pathway.

Further according to the invention the chambers are defined in acylindrical vessel which is sub-divided axially by spaced floors ordecks extending substantially over the entire cross-section of thevessel.

Preferably, the decks are spaced so that the layers of pulp supportedthereby do not exceed a maximum value at which the pulp at the bottom ofa layer contains a predetermined minimum gaseous content as outlinedabove.

In order to more effectively introduce oxygen at each transfer level andin order to minimize any tendency for compaction, the chamber betweenthe top two decks may be somewhat less in height than all subsequentchambers;

The chamber between the two top floors may thus be not more than 3.00meters and preferably not more than 1.20 meters high, although forapparatus handling pulps well fluffed and not below 20 percentconsistency this restriction need not apply.

Preferably the dividing wall extends over substantially the entireheight of I the chamber. Thus the spaces above each deck are dividedinto a plurality of separate upright pulp holding compartments, eachdeck being arranged for pulp to be transferred sequentially fromdifferent compartments above it to the space below it. The pulp from acompartment above a deck may be transferred to a compartment below thedeck and preferably, the pulp is transferred in a uniform manner withoutany hang-ups.

A plurality of dividing walls may be provided which radiate from acentral axis to provide the plurality of pulp holding compartments.

In one arrangement, the dividing walls are, stationary and the floors ofthe chambers rotatable.

In another arrangement the dividing walls are rotated and the floors arestationary. Means for rotation may comprise a shaft extending axially inthe pressure vessel, the floors or the dividing walls, as the case maybe, being mounted on the shaft for rotation therewith.

In an alternative arrangement said means comprises a shaft extendingaxially in the pressure vessel and at least one scraper blade, or rakearm provided above each floor and fast with the shaft to rotatetherewith.

Further according to the invention the lower regions of the dividingwalls are so located relative to the chamber floor as to direct the pulpin a compartment towards the outlet port with minimal transfer of pulpbetween compartments in the same chamber.

The outlet port may comprise a substantially wedge shaped aperture inthe chamber floor.

Preferably outlet ports of adjacent chambers are angularly off-set fromone another with respect to the axis, so that substantially all the pulppassing into the compartment is retained for the controlled residenceperiod.

Means may also be provided for fluffing pulp on introducing it to saidpathway. Preferably a rotary feeding device is provided for distributionof pulp above the upper floor.

The pressure vessel may be provided with a pulp inlet towards its upperend and with a pulp outlet towards its lower end. The vessel may furtherbe provided with one or more steam and oxygen gas inlets to maintain thetemperature and pressure at a selected value such as C and p.s.i. g.

The pressure vessel towards its outlet may be provided with means tore-unite the pulp into a single column which may be washed, diluted,cooled and discharged from the vessel in known manner.

An advantage of the apparatus is that the shaft may enter the vessel atthe bottom below the level of the diluted pulp in this way no glandsoperate in the gaseous phase thus simplifying the shaft sealing problem.

An added advantage is that the compartments are run substantially fullwhich is an important factor in equipment operating under pressure.

For a clear understanding of the invention three preferred embodimentswill now be described by way of example with reference to theaccompanying drawings in which:

FIG. 1 is a vertical sectional view of one embodiment of a pulp treatingapparatus according to the invention,

FIG. 2 is a cross-section on the line II-II in FIG. 1,

FIG. 3 is a cross-section on the line III-III in FIG. 1;

FIG. 4 is a cross-section on the line IVIV in FIG. 1;

FIG. 5 is a cross-section on the line V-V in FIG. 1;

FIG. 6 is a cross-section on the line VI-VI in FIG. 1;

FIG. 7 is a vertical sectional view of a second embodiment of a pulptreating apparatus according to the invention;

FIG. 8 is a cross-section on the line VIII-VIII in FIG. 7;

FIG. 9 is a cross-section on the line IX-IX in FIG. 7;

FIG. 10 is a crossasection on the line X-X in FIG. 7;

FIG. 11 is a cross-section on the line XIXI in FIG. 7;

FIG. 12 is a vertical sectional view with parts broken away of a thirdembodiment of a pulp treating apparatus according to the invention;

FIG. 13 is a cross-section on the line XIII-XIII in FIG. 12;

FIG. 14 is a cross-section on the line XIV-XIV in FIG. 12;

FIG. 15 is a cross-section on the line XV-XV in FIG. 12; and

FIG. 16 is a cross-section on the line XVI-XVI in FIG. 12.

Referring to FIGS. 1 to 6 reactor 1 comprises a cylindrical pressurevessel which is provided with a plurality of vertically spaced pulpsupport decks 2a, 2b, 2c and 2d in the form of spaced discs, mounted onshaft 3 which is located co-axially within reaction vessel 1 and whichis adapted to be rotatably driven by a motor (not shown). The number ofdecks may be increased to any convenient number and are not limited to 4as in this example.

A plurality of vertically extending, radially disposed dividing walls 5are located above each of the decks 2a, 2b, 2c and 2d to divide thechamber above each deck into a plurality of upright compartments 6disposed radially about shaft 3. The dividing walls 5 at the difierentlevels are vertically aligned to provide corresponding compartments 6 atthe different levels which are vertically aligned. The dividing walls 5above the deck 2b are about 1.20 meters high and the dividing walls 5above the decks 2c and 2d are about 1.30 meters high. The height of thechamber above deck 2a is more than 1.20 meters in order to be able toprovide the pulp layer of 1.20 meters in the chamber above deck 2b.

As can be seen from FIGS. 2 to 6 of the drawings, each deck is providedwith an aperture 7 which may for instance conform to the cross-sectionalconfiguration of the compartments 6, the apertures of the various decksbeing angularly off-set preferably by an angle greater than a butusually less than 2a where (1 corresponds to the angle subtended by onecompartment 6 at the axis. The angle is so adjusted that all the pulppassing into the compartment is retained for the controlled residenceperiod.

At its upper end, reaction vessel 1 is provided with a pulp inlet 8 anda rotary feeding device 9 which is mounted on shaft 3 and which isadapted to direct incoming pulp into the various compartments 6 aboveupper deck 2a. Device 9d is provided to fluff incoming pulp. At itslower end, reaction vessel 1 is provided with dilution nozzles 14 andwith a scraper and extractor 10 for discharging treated pulp throughoutlet 1 1. The reaction vessel 1 is further provided at its upper endwith oxygen and steam inlets 12 and at its lower end with oxygen inlets13.

In operation, heated pulp plus chemicals and oxygen plus steam areintroduced into reaction vessel 1 at its upper end through inlets 8 and12 respectively. The oxygen is admixed with the pulp by the fluffingdevice and then directed into compartments 6 above upper deck 2a byfeeding device 9. The pulp may be premixed with oxygen before enteringreaction vessel 1, if necessary. By rotatably driving shaft 3, the decks2a, 2b, 2c and 2d are rotated so that the aperture 7 in each deck passessuccessively underneath the compartments 6 above such deck. It will beappreciated that as each deck rotates, a batch of pulp equal in quantityto the layer supported by deck 2b drops out of each successivecompartment 6 above the deck into the corresponding compartment belowthe deck. Once the aperture 7 in a deck has passed a compartment 6 abovethe deck, the pulp in that compartment 6 above the deck remainssubstantially undisturbed until the aperture 7 reaches that compartmentagain during the next revolution of the deck. In this manner pulp istransferred compartment by compartment in stepwise fashion from theupper end of the pressure vessel 1 towards the bottom of the vessel fromwhere treated pulp is discharged through outlet 11. It will beappreciated that the pulp contained in reaction vessel 1 is divided intoa plurality of batches, on each of the decks 2a, 2b, 2c and 2a by thedividing walls.

The speed of rotation of shaft 3 and therefore, of the decks, isdictated by the total height of the superimposed layers of pulp inreaction vessel 1 and the required reaction time. It will be appreciatedthat in the above example during each revolution less an angle equal tothe angle of off-set of the decks, the pulp moves down about 1.20meters, i.e., the height of the pulp layer above deck 2b. Where, forexample, the total height of pulp in the reaction vessel is 15 meters;and the required reaction time is minutes, and the pulp is required tomove downwardly at a speed of 0.5 meters per minute, the shaft speedwould be 0.5/1.2 0.41 r.p.m.

It will be appreciated that the reaction time can be changed by changingthe speed of rotation of the shaft and thus of the decks. In that event,it may also be necessary to change the rate of pulp introduction intoand extraction from the reaction vessel for satisfactory continuousoperation.

Referring to FIGS. 7 to 11, 21 designates an elongated uprightessentially circular cylindrical vessel widened at its lower end at 22.The vessel is pressure-resistant in order to withstand an inner pressureof the order of 150 p.s.i.g. A continuous feed for cellulose pulp isprovided of which merely supply tube 23 connected to the side of thevessel top is shown. The vessel 21 is further provided at its lower endwith means for continuous discharge of treated pulp which comprise anoutlet conduit 24 opening into the center of the vessel base and havingassociated therewith a blow valve 25, and a rotary scraper 26 forforwarding pulp towards that outlet conduit, the scraper operating closeto the vessel base and over its entire cross-section. Pulp discharge isfacilitated by its dilution with water or filtrate supplied through aconduit 27 and spread by means of nozzles 28 located in the vessel, wellimmediately above the base thereof. The scraper 26 is attached to ashaft 29 which is driven by a motor 30 coupled to gear box 31. Theshaft, which passes through the vessel base extends along the entirelength of the vessel 21 and has its upper end joumalled in a bearing 32located inside the top of the vessel.

Over the greater part of its length the shaft 29 fonns a tube 33.Between the upper end of said tube and a cylindrical wall 33 attachedthereto there is formed an annular tank 34 to which washing liquidpreferably heated water, is supplied through a conduit 35 in a quantitycontrolled by the valve 36. Through conduits 37 and 38 in the tube 33and through cavities in the arms of the scraper 26 the washing liquid issupplied to spray tubes attached to the said arms which have nozzles 39through which the washing liquid is spread between a set of hollowannular screen bodies 40 and 41 having cylindrical screen faces. Bymeans of hollow radial arms 42 and outlets 43 the cavities of the saidscreen bodies communicate with an exterior conduit 44 for dischargingthe liquid displaced by the washing liquid which is screened off thepulp.

Connected to the upper end of the container is a conduit 45 for thesupply of oxygen and a conduit 46 with valve 69 for the supply of hotwater, e.g., of a temperature of 130 C, in controlled quantities. Theconduit 46 is connected to a conduit 47 extending through the tube 33and leading to one or more radially extending spreader tubes 48 havingorifices distributed so that the hot water is spread over the entirecrosssection of the container as evenly as possible. To the top of thecontainer there is also connected a circulation conduit 49 with a lowpressure pump 50 and a control valve 51. Through the last-mentionedconduit oxygen may be pumped into the conduit 52 and out through theorifices of the spreader tubes 48 together with the hot water.

inserted at different levels in the upper part of the container aredecks 55, 56, 57, 58 which are essentially horizontal but may besomewhat inclined towards the center. The decks are attached to theshell of the container and extend close to but are free from the rotarytube 33, where they are bordered by an upwardly directed cylindricalcollar 59. Sector-shaped apertures 65, 66, 67, 68 in the floors serve aspassages for pulp.

Close to the decks there are pulp feeding means which are shaped as rakeor scraper arms 70 attached to the tube 33 and extending radiallytherefrom. The scraper arms may be somewhat curved and/or turned or maybe shaped as shovels and are arranged to move at different levels in thepulp layers collected upon the decks so that in addition to feeding pulpthrough the sector-shaped apertures they also serve to effect a certainstirring of the pulp. The apertures of the decks are angularly displacedrelatively to each other so that the pulp dropping therethrough iscaught by the deck located directly below.

Before the pulp to be bleached with oxygen is introduced into the abovedescribed reaction vessel, it is subjected to a pre-treatment consistingin impregnation thereof with NaOH and preferably a catalyst, preferablyMgCO by immersion into a mixture thereof. Then the pulp is concentratedto a consistency of 12-30 percent but preferably below 20 percent andmost preferably above 16 percent. It is important that chemicals areuniformly mixed into the pulp. The temperature of the pulp during theimpregnation is preferably maintained at about C. By means of a highdensity pump the pulp is introduced into the vessel 21 and the pressureis maintained at about 150 p.s.i.g. and is additionally heated toapproximately C, by supplying fresh steam. While being introduced intothe vessel 21 the pulp is also loosened or broken up so that the pulpforms fiber bundles the greater part of which have a minimumcross-section size lying below 5 mm, preferably below 2 mm. The upperpart of the container is filled with oxygen gas at the above pressureand said gas penetrates into the whirling or falling loose pulp beforethese are collected upon the uppermost deck 55. Continued access ofoxygen to the interior of the pulp layer formed upon the deck 55 isfacilitated by the agitation by means of the scraper arms 70. This feedsthe pulp gradually towards the aperture 65, from which the pulp dropsfreely to the next deck 56. During said free gravity fall the pulp isspread again so that the oxygen has access to the spaces between theindividual particles of the pulp and can penetrate into the poresthereof. Moreover, the pulp is subjected to a re-arrangement which isfavorable for an even treatment of the same. In this manner the pulpmoves in a series of drop steps through an upper treatment zoneincluding the decks.

Below said zone the pulp is united into a coherent column which fills upthe central and lower part of the vessel. The level 71 is maintainedconstant by controlling the feed and discharge of pulp into and out ofthe vessel.

The spaces between the particles of the pulp column are, at the lowerpart thereof, completely filled with the washing liquid supplied throughthe conduit 47 and more or less incompletely filled at the upper partthereof. By means of the pump 50 oxygen is forced out through theorifices of the rotary spreader arms 48 and therefore, in the treatmentzone above said arm oxygen bubbles up through the pulp column. Theoxygen not absorbed reaches the gas space and is recirculated.

In the next lower zone the reaction continues between the finallysupplied oxygen and the pulp diluted with washing water. While using upessentially all oxygen in the pulp the bleaching reaction is finished inthis zone.

Finally washing of the pulp takes place in the lowest zone of thevessel. Washing liquid supplied through the nozzles 39 displaces thetreating liquid containing the reaction products. Such liquid, whichstill is of a temperature of about 130 C, is displaced out through thescreens 40 and 41 and the conduit 44, to a pressure releasing tank 90.From the top outlet 91 thereof steam and remaining oxygen, if any, arelet out and after condensing the steam, the oxygen may be returned tothe vessel 21. Liquid leaving through the bottom outlet 92 is evaporatedand transferred to a chemical recovery plant, preferably together withspent liquor obtained from the digestion of the pulp. The washed pulp iscooled and diluted with water supplied through the nozzles 28 and thepulp is discharged at the bottom of the container at a temperature belowboiling temperature and its pressure is released to atmospheric pressurewhile passing the valve 25.

By using the above described apparatus bleaching of pulp by oxygen cantake place at a high consistency and at a moderate pressure in excess ofatmospheric pressure with great efficiency.

In the final embodiment shown in FIGS. 12 to 16 the apparatus isidentical to that illustrated in FIGS. 7 to 11 except that the means forfeeding pulp through the apertures 65 to 68 of the decks 55 to 58 isdifferent.

In this arrangement feeding means are designed as radially extendingplane vertical vanes 101, 102, 103 and 104 having such a great heightthat they cover almost the entire space between the decks. The vanes areattached to the rotary shaft or tube 33 and extend to the vicinity ofthe cylindrical shell 21 of the vessel. A set of vanes is arranged aboveeach deck and in each set the vanes are equal, in shape and height, butare angularly disposed as is shown in FIGS. 13 to 16. Above each deckthe space is divided by said vanes into a number of sector-shapedcompartments 105, in which the pulp is received. Stirring of the pulp isthus avoided when the shaft turns although the entire pulp layer restingupon a deck is moved by the vanes in a horizontal circular path.

As each compartment 105 between the vanes 101 of the upper set reachesthe aperture 65 of the deck 55 the pulp batch in the compartment isstepwise emptied therethrough, the pulp dropping upon the next lowerdeck 56 and being collected in the compartments 105 between the vanes102 arranged above said deck 55. The pulp drops by gravity in a stepwisecascade so that the pulp is spread or loosened and oxygen in thecontainer has access and can penetrate evenly into the smaller spacesand pores of the pulp before the same forms a horizontal layer again.

The apertures of the decks are sector-shaped and preferably arecongruent to the sectors of the above mentioned compartments.Furthermore, as mentioned the sector-shaped apertures of adjacent decksare angularly displaced relatively to each other, so that the pulp dropsin a number of steps equal to the number of floors. The angulardisplacement is so chosen with regard to the direction of rotation ofthe shaft 33 that the compartments first pass over the aperture of thedeck below for emptying the compartments and then pass immediately belowthe aperture of the deck above the compartments for re-filling thereof.Therefore, the pulp will rest upon each deck for a retention periodcorresponding to a rotary movement of the shaft of at least about 270.

We claim:

1. A continuous method of treating a cellulose pulp with oxygen gasunder pressure comprising the steps of continuously introducing pulp ata consistency of l6 to 67 percent into the upper region of a verticallyelongated pressure vessel; heating the pulp; distributing the heatedpulp in well divided form in a series of separate layers individuallysupported within the pressure vessel at different levels along thelength of the pressure vessel; progressively transferring pulpdownwardly from one layer to the next in the series after a residenceperiod in each layer; contacting the pulp in each layer with oxygen gasat a partial pressure in excess of 3 bars absolute; restricting theheight of at least certain of the layers to a maximum value at whichcompaction of pulp under its own weight at the bottom of the layer islimited to a maximum value at which the pulp at the bottom of the layerhas at least a predetermined minimum oxygen gas content; andcontinuously withdrawing treated pulp from the lower region of thepressure vessel.

2. A method as claimed in claim 1, wherein the pulp is fluffed prior tobeing distributed in layers.

3. A method as claimed in claim 1, wherein the pulp is transferred fromone layer to the next in the series and away from the last layer in theseries under the influence of gravity.

4. A method as claimed in claim 1, wherein pulp is transferredsequentially in stepwise fashion from one layer to the next in theseries and away from the last layer in the series.

5. A method as claimed in claim 1, wherein the pulp in each layer issubdivided into a plurality of discrete batches which are transferredsequentially in stepwise fashion from one layer to the next in theseries and away from the last layer in the series substantially withoutintermixing with other batches.

6. A method as claimed in claim 5, wherein the support for each layer ofpulp is removed sequentially from underneath successive batches of pulpin the layer to permit the sequential transfer of successive batches ofpulp from one layer to the next under the influence of gravity, thetransfer of pulp from one layer to the next being controlled for a batchof pulp transferred from one layer to the next to occupy free space insaid next layer which has previously been vacated by a batch of pulptransferred away from said next layer.

7. A method as claimed in claim 1, wherein the pulp in each layer iscollected in a plurality of separate compartments as discrete batcheswhich are transferred sequentially in stepwise fashion from one layer tothe next in the series and away from the last layer in the seriessubstantially without intermixing with other batches, new batches ofpulp being collected sequentially in successive compartments in thefirst layer in the series to replace batches which are transferred fromthe first to the second layer in the series.

8. A method as claimed in claim 1, wherein the height of each layer withthe possible exception of the first layer in the series does not exceed3.00 meters.

53. A method as claimed in claim 7, wherein the height of each layerwith the possible exception of the first layer in the series does notexceed 1.20 meters.

10. A continuous method of treating a cellulose pulp with oxygen gasunder pressure comprising the steps of continuously introducing pulp ata consistency of 16 to 67 percent into the upper region of a verticallyelongated pressure vessel; breaking up the pulp into well divided form;heating the pulp; distributing the heated and well divided pulp in aseries of separate layers which are individually supported within thepressure vessel at different levels along the length of the pressurevessel, each layer of pulp comprising a plurality of discrete batchessequentially removing the support for each layer from underneathsuccessive batches of pulp in the layer to permit the sequentialtransfer in stepwise fashion of successive batches of pulp from onelayer to the next in the series and away from the last layer in theseries under the influence of gravity substantially without intermixingwith other batches; controlling the transfer of pulp from one layer tothe next to permit a batch of pulp transferred from one layer to thenext to occupy free space in the next layer which has previously beenvacated by a batch of pulp transferred from said next layer; restrictingthe height of each layer of pulp with the possible exception of thefirst layer in the series not to exceed 3.00 meters; contacting the pulpin each layer with oxygen gas at a partial pressure in excess of 3 barsabsolute; re-uniting in the lower region of the pressure vessel batchesof pulp which have been transferred away from the last layer in theseries; and continuously withdrawing treated pulp from the lower regionof the pressure vessel.

11. A continuous method of treating cellulose pulp with oxygen gas underpressure comprising the steps of continuously introducing pulp at aconsistency of 16 to 67 percent into the upper region of a verticallyelongated pressure vessel; breaking up the pulp into well divided form;heating the pulp; distributing the heated and well divided pulp in aseries of separate layers individually supported within the pressurevessel at different levels between the top and the bottom of the vessel;progressively transferring pulp downwardly in stepwise fashion under theaction of gravity from one layer to the next in series; restricting theheight of each layer of pulp with the possible exception of the firstlayer in the series not to exceed 3.00 meters; contacting the pulp ineach layer with oxygen gas at a partial pressure in excess of 3 barsabsolute; and continuously withdrawing treated pulp from the lowerregion of the pressure vessel.

2. A method as claimed in claim 1, wherein the pulp is fluffed prior tobeing distributed in layers.
 3. A method as claimed in claim 1, whereinthe pulp is transferred from one layer to the next in the series andaway from the last layer in the series under the influence of gravity.4. A method as claimed in claim 1, wherein pulp is transferredsequentially in stepwise fashion from one layer to the next in theseries and away from the last layer in the series.
 5. A method asclaimed in claim 1, wherein the pulp in each layer is subdivided into aplurality of discrete batches which are transferred sequentially instepwise fashion from one layer to the next in the series and away fromthe last layer in the series substantially without intermixing withother batches.
 6. A method as claimed in claim 5, wherein the supportfor each layer of pulp is removed sequentially from underneathsuccessive batches of pulp in the layer to permit the sequentialtransfer of successive batches of pulp from one layer to the next underthe influence of gravity, the transfer of pulp from one layer to thenext being controlled for a batch of pulp transferred from one layer tothe next to occupy free space in said next layer which has previouslybeen vacated by a batch of pulp transferred away from said next layer.7. A method as claimed in claim 1, wherein the pulp in each layer iscollected in a plurality of separate compartments as discrete batcheswhich are transferred sequentially in stepwise fashion from one layer tothe next in the series and away from the last layer in the seriessubstantially without intermixing with other batches, new batches ofpulp being collected sequentially in successive compartments in thefirst layer in the series to replace batches which are transferred fromthe first to the second layer in the series.
 8. A method as claimed inclaim 1, wherein the height of each layer with the possible exception ofthe first layer in the series does not exceed 3.00 meters.
 9. A methodas claimed in claim 7, wherein the height of each layer with thepossible exception of the first layer in the series does not exceed 1.20meters.
 10. A continuous method of treating a cellulose pulp with oxygengas under pressure comprising the steps of continuously introducing pulpat a consistency of 16 to 67 percent into the upper region of avertically elongated pressure vessel; breaking up the pulp into welldivided form; heating the pulp; distributing the heated and well dividedpulp in a series of separate layers which are individually supportedwithin the presSure vessel at different levels along the length of thepressure vessel, each layer of pulp comprising a plurality of discretebatches sequentially removing the support for each layer from underneathsuccessive batches of pulp in the layer to permit the sequentialtransfer in stepwise fashion of successive batches of pulp from onelayer to the next in the series and away from the last layer in theseries under the influence of gravity substantially without intermixingwith other batches; controlling the transfer of pulp from one layer tothe next to permit a batch of pulp transferred from one layer to thenext to occupy free space in the next layer which has previously beenvacated by a batch of pulp transferred from said next layer; restrictingthe height of each layer of pulp with the possible exception of thefirst layer in the series not to exceed 3.00 meters; contacting the pulpin each layer with oxygen gas at a partial pressure in excess of 3 barsabsolute; re-uniting in the lower region of the pressure vessel batchesof pulp which have been transferred away from the last layer in theseries; and continuously withdrawing treated pulp from the lower regionof the pressure vessel.
 11. A continuous method of treating cellulosepulp with oxygen gas under pressure comprising the steps of continuouslyintroducing pulp at a consistency of 16 to 67 percent into the upperregion of a vertically elongated pressure vessel; breaking up the pulpinto well divided form; heating the pulp; distributing the heated andwell divided pulp in a series of separate layers individually supportedwithin the pressure vessel at different levels between the top and thebottom of the vessel; progressively transferring pulp downwardly instepwise fashion under the action of gravity from one layer to the nextin series; restricting the height of each layer of pulp with thepossible exception of the first layer in the series not to exceed 3.00meters; contacting the pulp in each layer with oxygen gas at a partialpressure in excess of 3 bars absolute; and continuously withdrawingtreated pulp from the lower region of the pressure vessel.